In plug-in hybrid electric vehicles (PHEVs), the fuel vapor canister primarily adsorbs refueling vapors, as refueling and diurnal vapors are sealed within the fuel tank by a fuel tank isolation valve. An air intake system hydrocarbon (AIS HC) trap may capture hydrocarbons emitted by leaky injectors for from fuel that may puddle in intake. The AIS HC trap may also capture uncombusted fuel that is trapped within the engine cylinders themselves. An AIS trap is required for vehicles to be classified as practically zero emissions vehicles (PZEVs).
However, depending on the position of the cylinder intake and exhaust valves when the engine is shut off, the uncombusted fuel may migrate to either the engine intake or the exhaust manifold and may then escape to atmosphere. This may both increase a vehicle's emissions and cause a vehicle to fail emissions certification testing.
Previous solutions to this problem involve the utilization of secondary air injection methods to reduce the escape of uncombusted and partially combusted hydrocarbons to atmosphere at a subsequent vehicle-on event. Air may be pumped into the exhaust to cause the catalyst to heat up faster and burn the trapped hydrocarbons. However, secondary air injection adds additional hardware and complexity to the vehicle.
The inventors herein have recognized the above issues and have developed systems and methods to at least partially address them. In one example, a method for an engine is presented, comprising, following an engine-off event, positioning a first engine cylinder with an intake valve open, opening a canister purge valve; and purging contents of the first engine cylinder to a fuel vapor canister. In this way, uncombusted fuel that may otherwise be emitted from the engine during a prolonged diurnal soak may be stored within the fuel vapor canister, thus decreasing overall vehicle emissions. Following purging the contents of the first engine cylinder to the fuel vapor canister, each remaining unpurged cylinder may be sequentially positioned with an intake valve open, the canister purge valve opened, and then each unpurged engine cylinder may be sequentially purged to the fuel vapor canister. The engine may then be restored to a default, engine-off position. During the purging routine, the engine intake may be sealed from atmosphere by closing a throttle, and the cylinder contents may be purged by activating a vacuum pump coupled between the fuel vapor canister and atmosphere. In order to vaporize any residual fuel in the engine cylinders, the engine may be spun unfueled for a duration prior to purging the cylinders. In this way, the fuel vapor may be adsorbed by the fuel vapor canister. The engine exhaust may also be purged to the fuel vapor canister, either by positioning a cylinder with both an intake valve and an exhaust valve open, and/or by opening an exhaust gas recirculation valve.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.